Inkjet printing mechanisms use inkjet cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit or other mechanism that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the face of the printhead.
In the past, the inkjet printhead was carried back and forth across the page in a carriage attached to a belt that was driven by a drive pulley and carriage drive motor. Typically, the drive pulley was located at one end of the printzone, and an idler or tensioning pulley was located at the opposite end of the printzone. Several different belt and drive pulley systems have been used. One the more popular systems employs a toothed belt, similar to a timing belt in automobiles, which is driven by a pulley having mating teeth formed in the pulley's drive surface. The pulley teeth engage the belt teeth to provide a very reliable system that never slips. This tooth arrangement has a high tension ratio across the drive pulley, which yields a low belt tension requirement. The term belt tension refers to the static axial load or nominal tension in the belt to which the belt is stretched before use. Low belt tensions are preferred because higher belt tensions yield increased friction, higher motor heat, and wear. Moreover, with lower belt tensions both the motor and belt-tensioning pulley may be constructed without ball bearings, which reduces the overall system cost. Often, separate spring-biased, belt tensioning devices were used to provide a desired static belt tension, while also removing undesirable slack in the belt. Unfortunately, these belt tensioners increased the overall cost of the printing mechanism, not only in terms of additional component costs, but also in labor costs for assembly.
In general, the toothed belt drives have some inherent disadvantages. For example, the teeth do not transmit power smoothly when driving the carriage because the engagement and disengagement of the teeth produces a non-uniform driving force. Additionally, the belt tooth passing vibration occurs at frequencies that induce undesirable carriage velocity ripple. Moreover, these tooth engagement disturbances excited numerous noise sources within the printer, due to resonance which was concentrated in narrow frequency bands. Thus, printers using a toothed belt carriage drive system were perceived as being noisy, and a source of annoyance to consumers. Unfortunately, the belt tensioning devices mentioned above were unable to dynamically respond to these high frequency, rapid vibrations to provide adequate damping of this noise source.
To achieve accurate printing it is important to know or maintain an accurate positional relationship between the carriage and the media, with the printhead carriage moving smoothly across the media with minimum vibration to accurately locate each ink droplet of the image. As the number of dots per inch increases, the dot size has decreased, increasing the dot density to yield higher quality images, particularly in photographic images. One challenge in striving to achieve such improved image quality is the adverse impact of carriage vibrations. Consider now a situation where the carriage vibrates during printing over an entire image, the effect appears as a banding of lighter and darker areas of the image. Given the same vibration amplitude, the impact to an image formed of smaller dots is more adverse than to an image formed with the larger dots. In general, the smaller dot size and higher resolution of advancing ink jet printers require more accurate placement of dots to achieve expected image quality improvements. Any vibrations displacing the carriage relative to the media can potentially reduce printing accuracy. Typical sources of vibration are external vibrations which move the whole printer or scanner, and internal sources which stem from items coupled to the carriage, such as the carriage drive belt.
Another earlier carriage drive system employs a V-shaped belt driven by a pulley having a V-shaped groove around its periphery. While the V-belt drive systems exhibit improved acoustic properties and more consistent driving forces, unfortunately they have significant drawbacks. For instance, the V-belt drive system is susceptible to slipping when oil or other lubricants inadvertently contact the belt. The V-belts are inherently thick, and must be wrapped around a large diameter pulley, which made it necessary to use larger motor, since the pulley diameter could not be chosen to optimize motor performance. Moreover, the larger diameter pulley also increases the internal space required for the V-belt drive system within the printer. Another disadvantage of the V-belt drive is the low tension ratio across the drive pulley, which unfortunately induces high belt tension, leaving the belts susceptible to premature breakage. Thus, reliability of the V-belt drive systems is questionable. This high belt tension also increases friction in the V-belt system unless expensive ball bearings are used on the rotating components.
Another carriage drive system that has been proposed is a smooth belt which runs on a smooth pulley. Unfortunately, the smooth belt system is severely limited in the amount of power which it can transmit. In other words, as the driven load increases, for instance due to larger inkjet cartridges carrying greater supplies of ink, the smooth belts slip on the smooth pulleys. And, of course, this slippage increases if the smooth belt system is exposed to oil or other lubricating contaminants. Another system that has been proposed uses a smooth belt driven by a pulley having a drive surface coated with a grit material or having a knurled drive surface.
Thus, there exists a need for an inkjet carriage drive belt system which removes undesirable periodic belt tension vibration, and which may also eliminate the need for separate belt tensioning and slack removal devices, while providing an accurate, reliable carriage drive.